The coking of low value heavy petroleum streams, e.g., a vacuum resid, is a known process for upgrading such materials. In a common type of coking process, delayed coking, a heavy oil feedstock is heated rapidly in a fired heater or tubular furnace from which it flows directly to a large coking drum which is maintained under conditions at which coking occurs, generally with temperatures above 450.degree. C. under a slightly superatmospheric pressure. It is usual practice to employ two coking drums with one drum being charged with feedstock while product coke is being removed from the other drum.
In accordance with commonly assigned, copending U.S. patent application Ser. No. 07/779,657, filed Oct. 21, 1991, refinery-derived alkali-containing aqueous waste streams can be added to a delayed coking unit during coking to produce lighter, lower boiling products and an alkali metal-containing coke product. The waste streams can be those resulting from such refinery operations as Merox treating, caustic scrubbing, mercapfining and hydrogen sulfide removal from liquid and gaseous petroleum products.
U.S. Pat. No. 3,179,584 (Hamner et al.) discloses that the addition of a caustic material such as an aqueous solution of sodium hydroxide or potassium hydroxide to a heavy oil feedstock in a coking operation increases the amount of hydrogen formed during coking. The carbonate or oxide form of the alkali metal is also said to be useful.
U.S. Pat. No. 3,803,023 (Hamner et al.) describes a process for coking a heavy carbonaceous feed, e.g., a resid, in a coking zone in the presence of an alkali metal compound, e.g., the hydroxide, carbonate, sulfide, silicate or organic salts of potassium, sodium, lithium, rubidium and cesium, to provide an alkali metal-containing coke, gasifying at least a portion of the coke in a separate gasification zone in the presence of steam to convert at least a portion of the coke to a gaseous product and to obtain a partially gasified alkali metal-containing coke, passing the partially gasified coke to the coking zone and coking additional heavy carbonaceous feed in the coking zone where the resulting coke deposits on the partially gasified alkali metal-containing coke. The presence of alkali metal in the gasification zone results in increased gas yields. The gas contains over twice as much CO.sub.2, about half as much CO and less than half as much methane.
It is notable that '023 teaches the gasification of coke containing an alkali sulfide wherein the alkali sulfide is added to a heavy resid (coker feed) prior to coking. The alkali sulfide is not disclosed in '023 as being a component in an aqueous waste stream as disclosed in the instant invention. Furthermore, in the instant invention, residuum (coker feed) treated with water during the coking stage yields higher conversion during subsequent gasification of the coke than coke formed from residuum wherein only catalyst and no water is used to treat the residuum.
U.S. Pat. No. 2,626,892 (McCulley et al.) teaches the cracking of residual fractions containing an excess of salts, such as sodium sulfonates and naphthenates resulting from caustic treatment. Coke is formed by use of the McCulley process, the coke containing a salt residue. McCulley does not mention the presence in the residual fractions being cracked of sulfides or of any compounds which, upon gasification, would yield hydrogen sulfide. The presence of sulfides in the waste stream injected with the coke is a crucial element of the instant invention.
McCulley does not state that sulfides are inherently present in the McCulley reactant streams in any amount, either at a contaminant level or in operating amounts. McCulley does not add alkali compounds to the resid (coker feed) being treated. McCulley regards these compounds as being deleterious and seeks to remove these compounds from a petroleum fraction having an excess of alkali salts by cracking in a vessel to form a coke which carries the compounds from the cracked products. Furthermore, McCulley does not contemplate subsequent gasification of the coke. McCulley differs from Hamner '023 and the instant invention in that it considers alkali compounds to be deleterious and does not deliberately add them.